2. Mentha canadensis L. (corn mint)
M. arvensis L., misapplied
M. arvensis ssp. canadensis (L.) H. Hara
M. arvensis f. glabra (Benth.) S.R. Stewart
M. arvensis f. glabrata (Benth.) S.R. Stewart
M. arvensis ssp. piperascens (Malinv. ex
Holmes) H. Hara
M. arvensis var. villosa (Benth.) S.R.
Stewart
Pl. 436 i; Map
1964
Stems 15–80 cm
long, glabrous or sparsely to densely hairy with shorter and/or longer,
downward curved to spreading hairs, sometimes mostly along the angles. Leaves
short-petiolate, the petiole often narrowly winged. Leaf blades 1.5–8(–12.0) cm
long, lanceolate to ovate, elliptic or somewhat rhombic, broadly angled to
narrowly tapered at the base, tapered to a sharply pointed tip, the margins
sharply toothed, sometimes also short-hairy, the surfaces glabrous or sparsely
to moderately pubescent with short, loosely appressed hairs. Inflorescences of
axillary clusters, the subtending leaves similar in size to other foliage
leaves. Flowers almost always perfect. Calyces 2.5–3.2 mm long, glabrous, but
often with sessile glands (hairy elsewhere). Corollas 4–7 mm long, white to
lavender or pale pink. Nutlets 0.7–1.3 mm long, usually yellowish brown. 2n=96.
July–September.
Scattered nearly
throughout the state (most commonly in counties along the Missouri and
Mississippi Rivers), but apparently absent for the Unglaciated Plains Division
and uncommon in the western half of the Ozarks (U.S. [including Alaska];
Canada, Mexico, Asia; introduced widely). Bottomland forests, banks of streams,
rivers, and spring branches, banks of ponds, marshes, fens, marshes, and bases
of bluffs; also pastures, railroads, roadsides, and moist, disturbed areas;
sometimes emergent aquatics.
Mentha
canadensis is the
natural source of menthol, an important ingredient in cough drops, candies,
chewing gums, tooth pastes, shaving products, perfumes, and mentholated
cigarettes, among other uses. The species also is used medicinally. Currently,
India and China are the main producers of menthol, but synthetic production
also accounts for a significant share of the market (Hopp and Lawrence, 2007).
Mentha
arvensis and its allies
comprise a circumboreal polyploid complex within which a number of taxa have
been recognized as species, subspecies, forms, and hybrids (Harley and
Brighton, 1977; Tucker and Chambers, 2002; Tucker and Naczi, 2007). Tucker and
Chambers (2002) performed artificial crosses between selected taxa and analyzed
the cytology, phytochemistry, and morphology of the resulting hybrids. They
determined that North American and Asian populations previously attributed to M.
arvensis most likely represent an ancient, stabilized hybrid between the
Old World native diploid (2n=24) species, M. longifolia, and a
hexaploid (2n=76) population of the European M. arvensis in the
strict sense. They resurrected the Linnaean name M. canadensis for this
octoploid species. The American/Asian plants are the source of menthol, whereas
the related compound, pulegone, is the main ingredient in the essential oils in
most of the nine chemotypes of the European taxon (Tucker and Chambers, 2002),
which tends to emit a lavender-like fragrance from the crushed foliage.
Morphologically, true M. arvensis differs from M. canadensis
relatively subtly. According to Gleason and Cronquist (1991) and Tucker and
Naczi (2007), M. arvensis has less robust, less strongly ascending stems
and ovate to nearly circular leaf blades relatively uniform in size along the
stem (vs. progressively smaller toward the stem tip), those subtending flower
clusters tending to have more broadly rounded bases. However, Tucker and
Chambers (2002) noted that there was some overlap between the two taxa for the
leaf characters that they studied. Gleason and Cronquist (1991) further noted
that the European taxon is introduced in some northeastern states and provinces
(but not Missouri), with intermediates fairly common in the region in which
both taxa grow.
Mentha
×gracilis Sole (2n=54,
60, 72, 84, 96, 108, 120) (Pl. 436 g, h) represents a morphologically variable
series of sterile hybrids between M. arvensis and M. spicata.
In most of the earlier botanical literature, these have been called Mentha
×gentilis L. (which was based on a male-sterile clone of M. arvensis
rather than a hybrid) or M. ×cardiaca J. Gerard ex Baker (a later
name for M. ×gracilis) (Tucker and Fairbrothers, 1990; Tucker et al.,
1991; Tucker and Naczi, 2007). Two independently derived clones within this
hybrid complex known as American spearmint and Scotch spearmint are commercial
sources of spearmint (Tucker and Fairbrothers, 1990; Tucker et al., 1991),
although the parental taxon, M. spicata, also is an important source of
this compound. See the treatment of that species for further discussion of
spearmint. Mentha ×gracilis currently is known from six counties widely
scattered in the state. It is spread vegetatively and commonly found in
disturbed habitats such as ditches, railroads, and roadsides, but it also has
been reported from banks of streams and fens.